Integrating Sphere Aids Absolute Calibration of Lamps

Manufacturers of lamps, bulbs and other light sources use integrating spheres to measure in lumens the emitted visible light, or luminous flux, of their products. In addition to reporting this information to customers, manufacturers can gauge whether their products comply with US government standards that regulate energy input per lumen output.

NIST's 2.5-m-diameter sphere, manufactured by Labsphere Inc., establishes the absolute standard against which commercial spheres are calibrated. Lamp manufacturers use transfer lamp standards traceable to NIST to calibrate their integrating spheres. Courtesy of NIST.The National Institute of Standards and Technology (NIST) establishes the absolute standard against which these commercial spheres are calibrated. To do this, it uses an unusually large integrating sphere -- with a 2.5-m interior diameter -- that was designed and manufactured by Labsphere Inc.

"Lamp manufacturers rely on NIST to set the reference value for the lumen. They use transfer lamp standards traceable to NIST to calibrate their integrating spheres," explained Kevin Carr, Labsphere's director of product development.

The dimensions and other specifications for the sphere were stipulated by Yoshihiro Ohno, a physicist at NIST. He explained that, as a general rule, the larger the sphere, the smaller the errors in measuring luminous flux for different light sources. As a rough example, he said that calibrating a 1.5-m tubular lamp in a 2.5-m-diameter sphere against a small incandescent standard will produce half the error that would result from calibrating the same lamp in a 2-m sphere.

Two years ago, when NIST opened bidding on the project, Labsphere was the only company to respond. "There are not many manufacturers who can make integrating spheres this big," said Ohno.

"There were some pretty stiff specifications from Yoshi," Carr added. Ohno based those specs on perfect integrating sphere theory, which postulates the performance of an ideally spherical, evenly coated interior. He also set high standards for characteristics typically regarded as secondary, such as how well the two halves of the sphere closed, and the size and position of the baffle that allows researchers to see the intensity of interior illumination without seeing the light source.

The interior of NIST's sphere is coated with Labsphere's Spectraflect, a spray-on barium sulfate material that reflects light in the visible region of the spectrum. The design of the sphere allows the option of upgrading to Labsphere's Spectralon material if NIST decides to establish absolute standards for measuring the flux from ultraviolet light sources. Spectralon is a moldable, machinable thermoplastic.

The size of the sphere required Labsphere to manufacture it in sections and then reassemble it at NIST. Once it was assembled, Ohno tested the device's performance to fine-tune its ability to establish an absolute calibration for the lighting industry.